1. Technical Field
The present invention relates to an optical fiber cable. More particularly, the present invention relates to an optical fiber cable using a grooved spacer formed with SZ-spiral grooves, accommodating optical fiber tapes (ribbons), in its outer circumference along its longitudinal direction.
2. Description of the Related Art
An optical fiber is for example comprised of a fiber core, a cladding provided on the outer circumference of the core and plastic coating.
Such an optical fiber is in most cases used bundled with several other fibers in a tape form.
An optical fiber cable using a grooved spacer having SZ-spiral grooves in its outer circumference and accommodating optical fibers in those grooves is advantageous in that it is easy to take the optical fibers out from the grooves at the time of connection or terminal treatment. In this type of cable, the optical fibers used were optical fiber tapes. There have been known cables with a plurality of optical fiber tapes accommodated in the grooves stacked in the direction of depth of the grooves (Japanese Unexamined Patent Publication (Kokai) No. 2-83507 and Japanese Unexamined Patent Publication (Kokai) No. 5-203849) and cables with them accommodated in the grooves stacked in the width direction of the grooves (Japanese Unexamined Patent Publication (Kokai) No. 4-55803).
An SZ-spiral groove is formed continuously on the outer circumference of the spacer of the optical fiber cable in a periodically inverting wave shape along the longitudinal direction of the spacer. That is, the SZ-spiral groove is not formed as one-directional spiral groove (that is, an S-spiral type of Z-spiral type groove), but is formed in a wave shape periodically inverting along the outer circumference of the spacer. That is, the SZ-spiral groove periodically inverts in the direction of the spiral.
A spacer with such an SZ-spiral groove formed on its outer circumference has the advantage that it is easy to take out the optical fiber tape from the groove when making connections, performing terminal treatment, etc.
Japanese Unexamined Patent Publication (Kokai) No. 2-83507 and Japanese Unexamined Patent Publication (Kokai) No. 5-203849 disclose optical fiber cables where a number of optical fiber tapes are accommodated in grooves stacked in the direction of depth of the grooves.
Further, Japanese Unexamined Patent Publication (Kokai) No. 4-55803 discloses an optical fiber cable where optical fiber tapes are accommodated in an SZ-spiral groove with the optical fiber tapes stacked in the width direction (lateral direction) of the SZ-spiral groove.
An optical fiber tape is made by arranging a plurality of optical fibers in parallel on the same plane and then applying a common covering to form them into a tape shape. Therefore, when flexed in a direction causing the tape surface to bend, the individual optical fibers uniformly bend and therefore there is almost no increase in transmission loss. When flexed in a direction causing the side edges of the tape to bend in the plane of the tape (edgewise flexing), however, the optical fibers at the inside of the bend are subjected to strain in the compressive direction, while the optical fibers at the outside of the bend are subjected to strain in the tensile direction. This strain causes a large increase in transmission loss.
The occurrence of transmission loss will be explained in further detail here. If a groove for accommodating optical fiber tapes is formed as a one-directional spiral groove on the outer circumference of a spacer, the optical fiber tapes need only be accommodated so as to be wound around the cable in the one-directional spiral groove, so there is no large strain on the optical fiber tapes. If an SZ-spiral groove is formed in a wave shape on the outer circumference of the spacer, however, a plurality of optical fiber tapes are accommodated stacked in a certain direction with respect to the direction of the SZ-spiral groove, so there is always a portion to which a flexing motion in a direction causing the tape side edges to bend occurs in the optical fiber tapes. For example, in cables with the plurality of optical fiber tapes stacked in the depth direction of a groove as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-83507 and Japanese Unexamined Patent Publication (Kokai) No. 5-203849, the optical fiber tapes mainly receive a flexing motion in a direction causing the tape side edges to bend at the inverting portions of the SZ-spiral groove. Further, in a cable with a plurality of optical fiber tapes stacked in the width direction of the groove as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 4-55803, the tapes mainly receive a flexing motion in a direction causing the tape side edges to bend at the center portions between the inverting portions of the SZ-spiral groove (portions between one inverting portion and the next inverting portion). In both cases, transmission loss occurs.
In this way, an optical fiber cable of the type which uses a grooved spacer having an SZ-spiral groove and accommodating optical fiber tapes in those grooves is inevitably subjected to an excessive bending stress in the optical fiber tapes, so not only does the increase in the transmission loss of the optical fiber become greater, but also there is a problem in the point of the reliability in long-term use, thereby making commercial application difficult. An optical fiber cable having a spacer with an SZ-spiral groove, however, as mentioned above, has the advantage of easy removal of the optical fiber tapes from the groove at the time of connecting and terminal treatment of optical fiber tapes.